I picked up an old Onkyo HT-R380 amplifier that was stuck in standby / protect mode so pulled it apart to find out what the problem was.
This is a typical fault condition with a lot of old amplifiers as something fails and triggers one of many protection circuits that switch the unit off.
To get the boards out there are a lot of screws to remove as they’re attached to the back panel. You will might also need to remove a couple of screws that hold the back panel to the chassis to get the CPU board out. I read somewhere that the back panel on some amplifiers is used as a ground connection for different parts of the unit so if you remove it completely something may not get a ground or zero volt connection.
I’ve produced a YouTube video that takes you through how this Onkyo powers up and what causes it to switch into protect or standby mode.
There may be some errors in it, I determined how it works by looking at the circuit diagram and looking at some signals with an oscilloscope. Hopefully it will help you determine what the fault might be if you have a receiver with the same fault.
Note: There are high voltages present in these units along with microprocessor boards that don’t like high voltages so be careful when measuring voltages etc as if your multimeter or scope probe slips you could do a lot of damage.